Scintimammography was developed in the mid-1990s with the hope that it would become a useful diagnostic tool for breast cancer. Over the last 8 years, multiple studies have shown that while it has a very high overall sensitivity ( approximately 85%), this value drops to 50% or less for the detection of lesions C 10-15 mm in diameter. Dedicated small field of view gamma cameras show promise for the detection of small (<10mm) lesions in the breast. However, this technology is in its infancy and many technical questions on the optimal design of these systems remain. The purpose of this proposal is to develop and evaluate a clinically robust imaging system that will provide reliable detection of sub-10 mm lesions in the breast. The prototype gamma camera is a 20 cm x 20 cm semiconductor-based gamma camera utilizing modules of Cadmium-Zinc-Telluride. We plan to a) optimize the technical aspects of breast imaging, through improvement in energy resolution, collimation, and gantry design, b) perform clinical studies to determine the limits of lesion detection and the incremental value (if any) of a dual-head system, c) develop techniques for localization and biopsy of lesions by incorporating into the gantry design a breast ultrasound system that will permit simultaneous acquisition of scintigraphic and ultrasound data, and d) develop software to permit co-registration of the scintigraphic and ultrasound data. These latter goals are particularly important for lesions that are seen on scintimammography, but not visible on conventional mammograms. While many investigators have stated that a small detector is required for scintimammography, we believe that this is only the first stage in the development of a clinically useful system. Optimization of the gantry and performance characteristics of small field of view detectors, and providing a means of IocaIizing lesions, are critical to the migration of this technology from research into clinical practice.